Children with autism spectrum disorder (ASD) exhibit deficits in multiple domains including joint attention, imitation, and emulation. In typical development, these key social skills hinge on monitoring the activities and actions of others. Our recent study demonstrated that 20-month old toddlers with ASD attend less to activities than both age-matched typically developing (TD) toddlers and age- and cognitively-matched developmentally delayed (DD) toddlers, focusing instead more on non-social background elements such as toys. Given the cascading deficits that may arise when children are deprived of a vital route by which to learn from others, there exists a pressing need to better understand the developmental mechanisms leading to both typical and atypical activity monitoring. This proposal seeks to understand why toddlers with ASD monitor the activities of others less frequently by examining the social, perceptual, and developmental factors which may influence this ability in 20-month old toddlers with ASD and 14- and 20-month old TD toddlers. In regards to typical development, this proposal aims to chart the normative developmental changes in activity monitoring and to understand the bases for these changes, thus providing a context by which to understand limited activity monitoring in ASD. In regards to ASD, this proposal seeks to quantify to what extent limited activity monitoring in ASD is due to (1) orienting less towards the locus of the shared attention of others; (2) being less sensitive to motion cues; and (3) being more easily distracted from social activities by perceptual distractors. To answer these questions we present subjects with scenes depicting play interactions between two actors and modulate the following factors: (1) whether the actors are focused on the centrally shared area or mutually upon each other; (2) whether the scene is shown as a static image or a dynamic movie; and (3) the extent to which the background is cluttered with distractors such as toys. To clarify the impact of these factors, we also examine the relationships between patterns of attention in experimental conditions and socio-cognitive functioning in ASD. Finally, we augment traditional analysis of variance analytic approach with computational modeling and dynamic analyses in order to better understand the microstructure of activity monitoring in ASD and TD toddlers. Disruptions in activity monitoring at an age where rapid development and skill acquisition is occurring in TD children may critically impact the social experience of children with ASD. By understanding the factors contributing to limited activity monitoring in ASD, and by charting the changes in activity monitoring occurring in typical development, we hope to gain insight into those specific processes which may be altered in ASD. This work thus has the potential to lead to novel measures of outcome, to aid in the development of new tools for parsing the heterogeneity of ASD, and to inform the design of targeted treatment for ASD in early development.